This invention relates to a method for the application of cellular glass blocks to the outer surface of a spherical metal vessel, e.g., a storage tank for liquid ammonia or liquified natural gas.
Vessels of this type are generally expected to operate at temperatures from -50.degree. F. to +50.degree. F., with occasional high temperature cycles up to about 120.degree. F. Insulation is required so that the temperature and pressure within the vessel remains relatively constant. In the case of vessels intended for operation in the lower temperature ranges, properly applied insulation is recommended to prevent condensation and/or frost build-up under ambient conditions of high temperature and high humidity, e.g., 85.degree. F., 80% relative humidity.
In one conventional technique for applying cellular glass insulation blocks to spherical metal objects, the metal surface is cleaned and primed with an asphalt primer. Glass blocks are shaped to fit snugly to the shell and adjacent glass blocks and the surface of the blocks coated with the selected adhesive and the butt edges thereof coated with joint vapor sealant. The blocks are laid so that, when the blocks are pressed together, a complete vapor seal is formed with adjacent blocks.
In another conventional technique, the metal surface is blasted to gray metal and primed with an inorganic zinc primer. The glass insulation blocks are cut to fit the shell and adjacent insulation snugly and coated with urethanated asphalt premixed with catalyst and with vapor joint sealer at the butt edges.
In either case, when there is a surface irregularity, e.g., a weld joint, which causes a block to rock, the insulation block is scored to provide for good contact with the metal surface, to prevent breakage of the block and to eliminate voids which can lead to frost build-up.
The blocks are conventionally laid as follows:
1. The equator course is laid first and then seven or eight horizontal courses above the equator.
2. The next courses are applied vertically from points 90.degree. apart on the equator in such a manner that the courses meet over the top of the sphere.
3. There remain four pie-shaped sections in which blocks have to be cut to fit during application.
4. Starting at the bottom of the sphere, three to four courses of blocks are applied around the bottom pole.
5. The next courses are applied vertically from points 90.degree. apart running upward to the equator line.
6. There remain four pie-shaped sections in which blocks have to be cut to fit during application.
It will be obvious that there is substantial waste of glass insulation material, which must be cut to fit into the resulting 8 pie-shaped segments.
Generally, a tacky weather barrier mastic is applied over the insulation blocks. Small voids are first sealed with a sealing compound and larger voids with light weight fiberglass. Over the mastic is applied reinforcing cloth, which is overcoated with mastic to penetrate the cloth and achieve a good bond with the first coat of mastic.
Conventional procedures for the application of cellular glass blocks to metallic spheres may also require horizontal banding of the completed article with one to three aluminum or steel retaining bands.
Deficiencies of the existing methods include the requirement for different kinds of adhesives for the faces of the foam insulation blocks and the butt edges thereof, the effort required in applying aluminum retaining bands, difficulties in handling pre-catalyzed urethanated asphalt adhesives, and failure of the resulting adhesive bond after unacceptably short periods of time, with the resulting need to refurbish the insulation coating or to apply additional sealant to prevent condensation or frost accumulation.
It is an object of this invention to provide a method for applying blocks of cellular glass insulation to spherical metallic storage tanks in which
(1) the same material is used as adhesive and joint sealant, PA1 (2) the pot life of the adhesive is controllable, PA1 (3) the resulting insulation-coated sphere is more durable and resistant to premature failure than heretofore, PA1 (4) waste of insulating material is very low, and PA1 (5) retaining bands are not required. PA1 (a) cleaning the exterior surface of the vessel; PA1 (b) maintaining the vessel at a temperature of 80.degree.-100.degree. F.; PA1 (c) PA1 (d) repeating steps of (c) with successive blocks, each of which is butted firmly against adjacent previously-applied blocks, until the entire surface of the vessel is covered. PA1 (a) laying a first course of blocks with the long axis of each block parallel to the ground beginning at the top or bottom of the spherical vessel and extending from the top or bottom for a preselected distance of 20-30% of the radius of the sphere of the vessel; PA1 (b) laying a plurality of subsequent courses on either side of the first course, with the long axis of each block parallel to the ground, until an area of the sphere is covered corresponding to that circumscribed by the preselected distance of 20-30% of the radius of the sphere; PA1 (c) scribing a first circular line on the surface of the blocks laid in step (b), the line having a radius of the preselected distance of 20-30% of the radius of the sphere; PA1 (d) sawing through the blocks on the first circular line scribed in step (c) at right angles to the surface of the sphere; PA1 (e) laying subsequent courses of blocks with the long edges thereof parallel to the ground until a course is laid within the preselected distance of step (a) of the radius of the sphere from the extremity of the sphere opposite to that covered in step (a); PA1 (f) scribing a second circular line on the surface of the blocks laid in step (e), the second circular line having the same radius as the first circular line and originating at the extremity opposite to that covered in step (a); PA1 (g) sawing through the blocks on the second circular line at right angles to the surface of the sphere and PA1 (h) covering the remainder of the sphere as in steps (a) and (b).